As often as 40 times a day, “Stephen L.” has a sense that everything he is doing, he has done before. Once, to disrupt the constant drag of this déjà vu, he lurched into a wild dance in front of a washroom mirror. That was something he was sure he'd never done before.

"Zelda," a historian, watches as her great-granddaughter ascends to the ceiling and disappears. Another time she encounters horrible witches with protruding chins, and still another, she sees a teenage boy leaning on the hood of the car she rides in, his legs in the air. He balances unperturbed for five minutes as they travel down the road.

Both a man and his son hear voices. The son finds the voices' constant trivial instructions — move this glass, take that subway turnstile — annoying. The father takes a lighter view. Once he tried to follow their advice at the racetrack. It didn't work.

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In his newest book, "Hallucinations," Oliver Sacks manages to make something beguiling of the many ways our brains trick us. Sacks presents a field guide to our quirky operating system's powers of deception with storytelling that makes readers feel like medical insiders.

The book does not discuss the hallucinations most of us think of when we hear the word: the frightening voices and delusions that accompany severe mental illnesses such as schizophrenia. Sacks focuses, instead, on hallucinations far more common, those without the underlying menace of psychosis. His interest is the fleeting phantasm that can come before sleep, the strange creatures some see upon waking, the delirium of fever, the unusual visual symptoms that can precede migraines, the odd occurrences that for some presage epileptic seizure — such as Stephen L.'s déjà vu — and the voices some hear in moments of crisis.

Through the stories of patients and friends, Sacks can make his hallucinating subjects seem like the lucky ones, not so firmly shackled to pedestrian senses.

He apparently saw some advantage to their perspective. Throughout the book are Sacks' stories of his own hallucinations, some unbidden, but many sought through a surprising amount of drug experimentation in the 1960s. (Only surprising, perhaps, in light of the fact that he's Oliver Sacks, for Pete's sake!) At one point, he actually set aside Sundays for hallucinogen use. One psychedelic Sunday, he swallowed 20 tablets of the synthetic drug Artane. Then he made breakfast for people who were not there. Moments later, he grew excited to realize that his parents, who lived in Great Britain, were on a helicopter, and would in minutes be landing right beside his home in Topanga Canyon near Los Angeles. His disappointment when he realized the anticipated visit was a hallucination reduced him to tears. When he began seeing patients in a migraine clinic in 1966, he would schedule amphetamine weekends. That's when he read the 500-page, 100-year-old "On Megrim," in one weekend. It was the inspiration for his first book, "Migraine."

Often, Sacks' subjects talk about enjoying their nonthreatening hallucinations, especially when they recognize them as such. "Gertie C." was among the postencephalitic Parkinson's patients Sacks wrote about in "Awakenings." For years, she told him, she experienced pleasant hallucinations of sunlit meadows and childhood idylls. When she was prescribed L-dopa to control the Parkinson's symptoms, her hallucinations took on a sexual quality. "You surely wouldn't forbid a friendly hallucination to a frustrated old lady like me!" she told Sacks. She had enough control over these visions to permit herself one every evening. If a family member's visit threatened to delay her scheduled tryst, she would tell them she was expecting "a gentleman visitor from out of town," and she didn't want to keep him waiting.

But for others, hallucinations can be frightening and revolting, from the torment of reliving terror in post-traumatic stress disorder, to repellent hallucinatory smells before an epileptic seizure, to hearing voices that seem to bring on the seizure itself.

"Mrs. B" reports: "This is not like hearing a voice in a dream. It is a real voice." She would hear it off to the right, calling her name. She tries to ignore it. "[I]f I turn towards the voice I have a convulsion," she says. Still, she could never resist it.

Although Sacks couldn't have intended it as such, "Hallucinations" makes a substantial counterweight to the recent best-seller "Proof of Heaven" by neurosurgeon Eben Alexander. Alexander's near-death experience during a coma landed him on the cover of Newsweek. He employs his scientific and medical credentials to lend credence to his claims that he visited heaven while in a coma for seven days. And even the most convinced atheist may acknowledge that coming through this kind of ecstatic vision without some sense of the divine would be near impossible. But Sacks' book, by presenting the many ways in which our brains routinely produce miracles, create ghosts, invent alien abductions and generate ecstatic religious feelings, suggests the logical explanation that the much-publicized neurosurgeon explains away. And Sacks isn't even arguing the point.

In Charles Bonnet syndrome, for instance, the brain fills in imaginatively when visual impairment reduces what one sees. Depending on the nature of the hallucination, Sacks writes, these visions can be frightening, comforting or fascinating. The late acclaimed poet Virginia Hamilton Adair found inspiration in them when glaucoma took her eyesight, referring to "the angel of hallucinations."

The human brain can pull a similar stunt with prisoners in monotonous confinement, sailors long at sea, long-distance truckers traversing the interstate or pilots during long flights (Talk about frightening hallucinations — for the passengers).

So-called ecstatic seizures can completely alter religious belief. Author Fyodor Dostoyevsky lived with ecstatic seizures of this nature, as did some of the characters in his novels. Sacks relates the story of a bus conductor who had an ecstatic seizure while collecting fares. A 1970 paper on the subject relates "He collected the fares correctly, telling his passengers at the same time how pleased he was to be in Heaven. … He remained in this state of exaltation, hearing divine and angelic voices, for two days." Afterward, he remained religious. Three years later, however, three seizures in three days brought similar elation, only this time he was converted to atheism.

And maybe this malleability is the most astonishing thing. That our beliefs, our worldview, and even what we think of as our immutable selves, are all subject to the legerdemain of our very clever brains.